Engine brake retarders of the compression release type are believed to be well known in the art. These devices may be referred to as an engine brake or engine retarder, but regardless of the name, the theory of operation is basically the same. In general, such engine retarders are designed to open the exhaust valves or a special retarder valve of an internal combustion cylinder near the end of the compression stroke. As a result, the work done in compressing the intake air is not recovered during the expansion stroke, but rather is dissipated through the exhaust (and cooling) systems of the engine.
In a typical prior art engine retarder cycle, the exhaust valves, or a dedicated retarder valve for the cylinder is opened near the end of the compression stroke (approaching top dead center) and is held open at least throughout tile expansion and exhaust strokes. By opening the valve near the end of the compression stroke, the compressed air in the cylinder is bled out of the cylinder so that it will not apply as much pushing force against the cylinder during the expansion stroke. However, such an engine retarder cycle exhibits several problems. For example, by waiting until the end of the compression stroke to open the retarder valve, considerable pressure has built up within the cylinder which must be overcome by the circuit which opens the valve, thereby producing substantial mechanical loading upon the engine. Furthermore, because the prior art retarder valve remains open during the expansion stroke, a back flow of air from the exhaust manifold into the cylinder is created during the expansion stroke, which creates a force tending to push the piston down, thereby creating negative retarding work. This is obviously the opposite of the intended effect of the engine retarder cycle.
There is therefore a need ill the prior art for all improved engine retarder cycle which will reduce mechanical loading upon the engine caused by opening of the exhaust or retarder valves and which will reduce or eliminate the negative retarding work produced during the expansion stroke. The present invention is directed toward meeting these needs.